Patna: Investigators probing the CBSE Class X mathematics question paper leak have zeroed in on a suspect from Nalanda, who, they believe, is the mastermind of the racket.

"The kingpin of the CBSE Class X question paper leak hails from Nalanda," said an investigating officer who did not wish to be named. "We can't share more details about him as it will affect the investigation."

The suspect is on the radar of Jharkhand's Chatra police, who nabbed two students of Class X from Supaul district on Monday evening.

The two students, said to be cousins, are understood to have told their interrogators that they had received the question paper on WhatsApp a day before the examination. The question paper sent to the students matched with the original ones.

Both the suspects had appeared for the examination from a Patna school. "We had escaped to Supaul on the suggestion of the kingpin of the racket, who is evading arrest," an investigating officer quoted one of the two suspects as saying. Two other students were taken into custody from Patna last week.

The suspect from Nalanda had allegedly obtained the mathematics question paper from Chatra on his mobile phone and this was provided to the students of a private lodge at Ram Krishna Nagar in Patna.

"The students were confined in a room a couple of hours ahead of the examination where some teachers told them about the (probable) questions. The students were then straightway taken to the examination centre just five minutes before the start of the test so that the matter couldn't get leaked," the officer said, quoting one of the student suspects.

Chatra superintendent of police Akhilesh B. Variar said all the arrested persons, including the students, were being questioned. "Four of the suspects have been taken into custody from Bihar," he told The Telegraph over phone on Wednesday.

Nalanda, the home district of chief minister Nitish Kumar, had been the epicentre of the entrance exam leak scandal that had rocked the country in the early 2000s. The CBI had then arrested Nalanda native Ranjeet Singh a.k.a. Ranjeet Don, who was allegedly involved in the leak.

The name of another Nalanda resident, Sanjiv alias Guruji, figured in the leak of question papers of the clerical grade examination last year.